Function count 

pub fn count<T: SqlType + SingleValue, expr>(
    expr: expr,
) -> count<T, <expr as AsExpression<T>>::Expression>
where
    expr: AsExpression<T>,

Creates a SQL COUNT expression

As with most bare functions, this is not exported by default. You can import it specifically as diesel::dsl::count, or glob import diesel::dsl::*

Window Function Usage

This function can be used as window function. See WindowExpressionMethods for details

Aggregate Function Expression

This function can be used as aggregate expression. See AggregateExpressionMethods for details.

Examples

Normal function usage

assert_eq!(Ok(1), animals.select(count(name)).first(connection));

Window function

assert_eq!(
    Ok(1),
    animals
        .select(count(name).partition_by(id))
        .first(connection)
);

Aggregate function expression

assert_eq!(
    Ok(1),
    animals
        .select(count(name).aggregate_distinct())
        .first(connection)
);